The present invention relates to a method for polishing thin TiC, Ti(C,N) or TiN coatings, applied on, e.g., cutting tools, to an extremely high surface finish using the electropolishing technique.
Thin wear resistant coatings of one or more layers of TiC, TiN, Ti(C,N) and/or Al.sub.2 O.sub.3, are commonly applied on cutting tools and wear parts in order to increase their abrasive and chemical wear resistance. These coatings typically have a total thickness of 1-20 .mu.m and are applied using chemical vapor deposition (CVD), physical vapor deposition (PVD) and/or related techniques. The surface roughness of the coating after deposition depends on the roughness of the surface coated, on the total coating thickness and on the type of coating applied. In general, the surface of the coating will have at least the same roughness as the initial coated surface, the roughness will increase with coating thickness and a coating containing a layer of .alpha.-Al.sub.2 O.sub.3 will be rougher than one containing .kappa.-Al.sub.2 O.sub.3 or Ti-comprising layers only.
One particularly interesting family of coatings is illustrated in FIGS. 1a-b. Excluding some very thin bonding layers, the coating has an inner layer of Ti(C,N) deposited onto a cemented carbide cutting tool insert, an intermediate layer of .alpha.-Al.sub.2 O.sub.3 and a top layer of TiN. As deposited, this coating has unacceptable surface roughness, originating mainly from the rough .alpha.-Al.sub.2 O.sub.3 layer. This leads both to inferior performance and to a brownish, rather unattractive color of the insert. A smooth top layer of TiN generally has a shiny golden color which is sought for cosmetic reasons. Today these problems are avoided either by using thermodynamically less stable .kappa.-Al.sub.2 O.sub.3 instead of .alpha.-Al.sub.2 O.sub.3, by mechanically polishing the .alpha.-Al.sub.2 O.sub.3 layer before depositing TiN or by mechanically polishing the TiN layer. The first method in many cases leads to inferior performance. The second method is an expensive two-step deposition process and the third method does not render the desired shiny golden color.
Electrolytic smoothing or deburring is a commonly employed technique, especially for metallic materials. Two well-known processes are called electrochemical deburring and electropolishing. U.S. Pat. No. 4,405,422 discloses methods for electrolytic deburring of copper or copper alloys and U.S. Pat. No. 4,411,751 of steel or aluminum alloys. In U.S. Pat. No. 5,591,320 and Swedish Application No. 9602278-5, which corresponds to U.S. Ser. No. 07/556,952 now U.S. Pat. No. 5,200,311 methods for edge rounding and edge rounding and complete insert surface finishing, respectively, of cutting tool inserts by electropolishing in an electrolyte containing different amounts of perchloric (HClO.sub.4) and/or sulphuric (H.sub.2 SO.sub.4) acid in methanol are presented. Common for all these methods is that they are designed to produce smooth edges essentially without depth effect, each on a specific class of materials, and that they are applied prior to any coating process. Thus, any roughness originating from the coating itself is not eliminated.